"I want you."
Fun fact: Te quiero can also mean "I love you." But not as strongly as "te amo." Something you say to family and friends you care about.
This app do not explains the grammar behind any sentences. Request you to show the logical grammar as well to understand is properly.
If you're willing to do some research it really shines light on some subjects, you're primary source of learning shouldn't just be Duolingo.
If you use the desktop site, when you click on the topic and it says 2/6 or whatever, below it is a lightbulb icon... it will give you an overview of the grammar.
I remember the song bailamos by Enrique for this one.. te quiero.. amor mio... Esta noche bailamos
"ti" is an indirect object - I threw the ball to you
"te" is a direct object - I love you (direct objects are the recipient of the verb action)
The fact that "Te quiero" means "I want you" and "I love you" is confusing...
"Yo os deseo", you mean? "Os" is the second person, plural form: Os deseo (a vosotros), it is like the "ustedes", just more informal.
Te quiero means I love you. I want you to do something for me. Quiero que hagas algo para/por mi.